Sf9 cells

I am starting to culture Sf9 cells and am growing them in Grace's Insect
Medium, supplemented, +10%FCS + gentamycin. According to invitrogen it
should be possible to slough the cells off the plastic when it's time to
passage them. But the cells I have stick like crazy and do not want to
come off that way. Tapping the flask does not help. I've tried
scraping them off gently but the viablity (by trypan blue) is not great
65-70%). I would like better viability before going on to the virus
infection step.
Any suggestions?
TIA
Allison